The present invention generally relates to spread spectrum communication systems, and in particular to spread spectrum communication systems which can be used for a wireless microphone, a paging system, a transceiver, a remote control system and so on.
In a spread spectrum communication system, a pseudo-noise (hereafter simply referred to as PN) signal which is a sequential noise-like signal is used for spreading a spectrum to broaden the frequency band of a transmission signal by using a signal other than an information signal to be sent, and for narrowing the spectrum (despread) to obtain the original transmission signal from the band-broadened signal. In this case, a need exists such that a PN signal used in a reception part must have the same pattern as and must be synchronized with a PN signal used in a transmission part. In order to satisfy this requirement, it is necessary to supply clock signal synchronized with each other to the transmission and reception parts and to therefore generate PN signals which have the same pattern and are in synchronization with each other.
Referring to FIG. 1 which shows an example of a conventional spread spectrum communication system, a signal derived from a signal source 10 in a transmission part is modulated by a primary modulator 11 and is supplied to one of inputs of a multiplier 12a. The other input of the multiplier 12a is supplied with a PN signal generated by a PN signal generator 17a. Both the input signals of the multiplier 12a are multiplied. A multiplied result which corresponds to a spread transmission signal is supplied to an antenna 13a. The transmitted signal is passed through an antenna 13b in a reception part and is supplied to one of inputs of a multiplier 12b and a clock signal extracting circuit 16. The other input of the multiplier 12b is supplied with a PN signal generated by a PN signal generator 17b. The received signal is subjected to the despreading process by the multiplier 12b. The despread signal is supplied to a demodulator 14, and is demodulated thereby. The demodulator 14 generates the demodulated transmission signal.
In the above operation, the PN signal generated by the PN signal generator 17a must have the same pattern as and must be synchronized with the PN signal generated by the PN signal generator 17b. To meet this requirement, the PN signal generator 17a in the transmission part is provided with a clock signal generated by a clock signal generator 15, and in the reception part, the clock signal is extracted from the received signal by a clock signal extracting circuit 16, and the extracted clock signal is supplied to the PN signal generator 17b.
FIG. 2 shows another system configuration of the conventional spread spectrum communication system. In FIG. 2, constituents identical to those in FIG. 1 are denoted by reference numerals identical to those therein. A clock signal generator 20 generates a clock signal of a frequency fc, which is supplied to the PN signal generator 17a in the transmission part and the PN signal generator 17b in the reception part.
FIG. 3 shows a conceivable system configuration for the spectrum communication system. In FIG. 3, the same constituents as those in the previous figures are denoted by the same reference numerals as those therein. An exclusive clock signal transmitter 18 sends a clock signal through an antenna 18a. The sent clock signal is received by a clock signal receiver 19a in the transmission part through an antenna 21a, and is also received by a clock signal receiver 19b through an antenna 21b in the reception part. The clock signal receivers 19a and 19b supply the clock signals identical to each other to the PN signal generators 17a and 17b, respectively.
However, the system of FIG. 1 has a disadvantage that it is difficult to extract the clock signal from the spread signal in the reception part. The sytem of FIG. 2 has a disadvantage that transmission lines to be laid out on the ground are needed for supplying the clock signals to the transmission and reception parts, and therefore various limitations due to the use of the transmission lines exist. The system of FIG. 3 is impractical because it is necessary to install the exclusive clock signal transmitter 18 specifically designed for sending the clock signal which requires an exclusive transmission frequency to be added in the frequency spectrum, and the clock signal receivers 19a and 19b for receiving the clock signal from the clock signal transmitter 18.